238 



Ihal Hcliocidaiis vvijlhvoiivumma has such a larva, aUhouf»h with only one 

 ciliated band, might \w adthued Iu-it as proving the occiirrcnce of this 

 larval ty|)o also in lu'hiiioids. wiu'ii' it was otlierwise unknown. Bui il is 

 a very strong objection that in the whole class of the Asleroidea not one 

 case of larva' with ciliated rings is known, neither is anything correspond- 

 ing to the |)iipa-stage known in tliis class, wliic li is otherwise one of the 

 more primitive of l"".chino(lernis, at any rate more i)rimitive than Ophiu- 

 roids and I^ciiinoids. Another ini|)ortanl fact is this, that in all the cases 

 where larva' wilh ciliated rings occur, the eggs are large and yolk-laden. 

 This evidenll} means that the larvae with ciliated rings, developing from 

 such eggs, are modirK'd in accordance with the fact that they have food 

 enough in store in the yolk and therefore need not trouble with catching 

 food. The ciliated rings of these larvae decidedly would serve very badly 

 as food-gatherers. If that type of larvae were really the primitive form 

 we should have the remarkable fact here of an organism having arisen 

 evidently unable to subsist by its own means. The fact that the stage 

 with the ciliated rings is never indicated in the beginning of the larval 

 development, but always at its end in those forms which have larvae of 

 the typical ])elagic shape, is also very hard to understand on the assump- 

 tion that the form wilh the ciliated rings is the more i)rimitive. Also the 

 fact Ihal in the vivij)arous Opliioiwtus licxdclis the end)ryos develop into 

 larvae corresponding exactly to the generalized, {)rimitive type of Flchi- 

 noderm-larvae, is of considerable importance in this connection; this larva 

 decidedly need.s no special adaptation to pelagic life, and it is hard to see 

 why it should adopt this form instead of that with the ciliated rings, were 

 il not of j)hylogenetic importance. 



In my opinion there is then no doubt tiiat the larva with a simple 

 circumoral band, as it is found in the younger stages of all the four 

 main larval ly|)es of l^chinoderms, is the |)rimitive form, and that il 

 represents a true phylogenetic stage in the ancestry of l-Lchi- 

 noderms — that is to say, the larva in its simplest type, the Dipleii- 

 lula. Of course, the larvae of recent Echinoderms, with their more or 

 less highly specialized characters, do not represent ancestral types of the 

 various classes of the Echinoderms, Hipinnaria of the Asteroids, Echino- 

 pluleus of the Echinoids etc. They represent special adaptations of tlie 

 original pelagic ancestral form, having been modified along wilh I he 

 adults, so as to form groups corresponding with the natural groups of the 

 adults, the result being that there is a larval classification exactly parallel 

 with thai of the adults. But the primitive type, the Diplciinda must, I 

 have no doubt, represent the organism from which the whole of the 

 Echinoderm stini developed. 



